Freedom of information campaigners say the regulator takes too long to decide to release documents to the public, undermining efforts to open up official files.

The public has to wait more than a year and a half on average before the information commissioner delivers a decision, according to figures. A decision about gun-related crime in Yorkshire took three years and 10 months.

The commissioner's staff take eight months on average before they even start investigating a complaint. One person waited nearly two years for the commissioner to begin an investigation into their complaint, concerning a refusal to release information about student loans.

Criticism by the Campaign for Freedom of Information is levelled mainly at Richard Thomas, who was information commissioner for seven years until last month. He has been replaced by Christopher Graham, the former director-general of the Advertising Standards Authority.

Maurice Frankel, the campaign's director, said the delays were "sufficiently serious and widespread to represent a major threat to the Freedom of Information Act's effectiveness and public confidence in it". He said: "A delay of two to three years or more in reaching a decision, as happens in over a quarter of cases means that even if the information is ultimately disclosed, it may no longer be of interest or use to the requester.

"Requesters ... may be so frustrated that they become reluctant to use the act again or to complain to the information commissioner about refusals."

The information commissioner's job is to promote the Freedom of Information Act and decide whether public bodies are entitled to keep requested information secret.

Last month, Thomas said a lot of information had been disclosed since the act came into force in 2005, adding that it was now "a permanent fixture and core part of the fabric of public life". He said the act's role in helping to get details of MPs' expenses published had "cemented FoI's reputation as a success story".

Delays highlighted by the campaigners include decisions about Foreign Office documents on the number of civilian deaths in Iraq after the 2003 invasion, and compensation payments to miscarriage of justice victims. The information commissioner has a backlog of 1,300 complaints from the public to resolve.

Last night, a spokesman for the information commissioner said he would continue to make improvements, but that more people were using freedom of information and so more complaints were being lodged.

He said the commissioner did not have enough funding. Government bodies are also frequently criticised for taking a long time to release documents.